1. Field of the Invention
The present invention relates to a fuel cell system for supplying hydrogen gas produced from a mixture liquid composed of liquid fuel and water to an anode electrode which constitutes a fuel cell, and a method for regulating the liquid fuel for the same.
2. Description of the Related Art
The fuel cell stack, which is constructed, for example, by stacking a plurality of fuel cells each comprising an anode electrode and a cathode electrode disposed opposingly with a solid polymer ion exchange membrane interposed therebetween, the fuel cell being interposed between separators, has been developed, and it is being practically used for a variety of applications.
Such a fuel cell stack is designed as follows. That is, for example, a reformed gas (fuel gas), which contains a hydrogen-containing gas produced by reforming a mixture liquid (aqueous methanol solution) composed of water and liquid fuel such as methanol, is supplied to the anode electrode. An oxygen-containing gas (air or gas containing oxygen) is supplied to the cathode electrode. Thus, the hydrogen gas is ionized, while it flows through the solid polymer ion exchange membrane. Accordingly, electric energy is obtained at the outside of the fuel cell.
Various fuel cell systems has been suggested, which have such a structure that a predetermined amount of liquid fuel is mixed with a predetermined amount of water, and an obtained mixture liquid is supplied to a reformer. For example, a reforming raw material liquid supply apparatus is known, as disclosed in Japanese Laid-Open Patent Publication No. 8-91804. In this conventional technique, a reforming raw material liquid storage tank for storing a mixture liquid of methanol and water is arranged under a methanol-storing tank for storing methanol. The reforming raw material liquid storage tank is provided with a measuring means such as a specific gravity sensor, a concentration sensor, and a density sensor for measuring the mixing ratio of methanol and water. The opening degree of an automatic valve provided at a lower portion of the methanol storage tank is regulated depending on the mixing ratio of the reforming raw material liquid measured by the measuring means. Accordingly, the mixing ratio of methanol and water is adjusted to have a predetermined value in the reforming raw material liquid storage tank.
However, in the case of the conventional technique described above, the water, which is discharged from the fuel cell, is introduced into the reforming raw material liquid storage tank from a recovered water passage via a water separator. For this reason, the amount of water in the reforming raw material liquid storage tank is considerably increased, when the fuel cell is continuously operated. Therefore, in order to maintain a constant mixing ratio of the reforming raw material liquid, it is necessary to supply a large amount of methanol from the methanol storage tank to the reforming raw material liquid storage tank. As a result, the following problems are pointed out. That is, the amount of consumption of methanol is increased, and a considerably large size is required for the reforming raw material liquid storage tank itself for storing the reforming raw material liquid.
A principal object of the present invention is to provide a fuel cell system which makes it possible to highly accurately maintain the mixing ratio of liquid fuel and water, and realize a small size and simplification of the system with ease.